1. Field of The Invention
The present invention is concerned with catalyst useful for the treatment of gases to reduce contaminants contained therein. More specifically, the present invention concerned with improved catalysts which may function as oxidation catalysts and/or as catalysts of the type generally referred to as "three-way conversion" or "TWC" catalysts. Whereas oxidation catalysts have the capability of catalyzing reactions such as the oxidation of hydrocarbons and carbon monoxide, TWC catalysts are polyfunctional in that they have the capability of substantially simultaneously catalyzing both oxidation and reduction reactions, such as the oxidation of hydrocarbons and carbon monoxide and the reduction of nitrogen oxides. Both types of catalyst find utility in a number of fields, including the treatment of the exhaust gases from internal combustion engines, such as automobile and other gasoline-fueled engines.
2. Background and Related Art
Emissions standards for unburned hydrocarbons, carbon monoxide and nitrogen oxide contaminants in vehicle and other engine exhaust gases have been set by various governments and agencies. In order to meet such standards, so-called catalytic converters containing separate oxidation catalysts or separate oxidation and reduction catalysts, or a TWC catalyst, are emplaced in the exhaust gas line of internal combustion engines to promote the oxidation of unburned hydrocarbons ("HC") and carbon monoxide ("CO") and the reduction of nitrogen oxides ("NO.sub.x ") in the exhaust gas. If the engine operation is too rich in fuel to inherently provide sufficient oxygen in the exhaust gas to oxidize HC and NO, it is known to introduce oxygen into the exhaust gas as required. The known use of two separate catalyst beds in series, one to promote oxidation of HC and CO and the other to promote reduction of NO.sub.x, can be replaced by a single bed catalyst which substantially simultaneously promotes both oxidation and reduction as described above. However, such TWC catalysts usually require that the ratio of air to fuel ("A/F ratio") introduced into the engine whose exhaust gas is being treated be at, or within a narrow deviation from, the stoichiometric A/F ratio in order to achieve good efficiencies of conversion of all three classes of pollutants, i.e., HC, CO and NO.sub.x to innocuous substances, that is, to carbon dioxide, water and nitrogen.
A great deal of effort has been expended in an attempt to economically produce oxidation catalysts and TWC catalysts which exhibit good activity and long life in promoting the conversion of the HC and CO pollutants (oxidation catalysts) and the HC, CO and NO.sub.x pollutants (TWC catalysts) even when the pollutants are contained in very small quantities in the gas stream being treated. For this purpose, catalysts comprising one or more platinum group metals distended upon a high surface area, refractory oxide support are well known in the art. The support may comprise a high surface area alumina coating carried on a carrier such as a monolithic carrier comprising a refractory ceramic or metal honeycomb structure, as well known in the art. The carrier may also comprise refractory particles such as spheres or short, extruded segments of a refractory material such as alumina.
Thus, typical catalyst compositions comprise a minor amount of platinum or palladium, preferably including one or more of rhodium, ruthenium and iridium, especially rhodium, as a platinum group metal component dispersed on a high surface area alumina material. The catalytic activity of the material is enhanced by dispersing the catalytically active platinum group metal components on a very high surface area support layer. The catalytically active materials dispersed on the activated alumina may also contain one or more base metal oxides, such as oxides of nickel, cobalt, manganese, iron, rhenium, etc., as shown, for example, in O. D. Keith et al U.S. Pat. No. 4,552,732, Such high surface area alumina materials, loosely referred to in the art as "gamma alumina" or "activated alumina", typically exhibit a BET surface area in excess of 60 square meters per gram ("m.sup.2 /g"), often up to about 200 m.sup.2 /g or more. Such activated alumina is usually a mixture of the gamma and delta phases of alumina, but may also contain substantial amounts of eta, kappa and theta alumina phases.
A common deficiency associated with supported catalyst systems is thermal degradation of the catalyst refractory oxide support from extended exposure to high exhaust gas temperatures of the automotive or other internal combustion engine. In a moving vehicle, for example, exhaust temperatures can reach 1000.degree. C., and such elevated temperatures cause the support material to undergo a phase transition with accompanying volume shrinkage, especially in the presence of steam, whereby the catalytic metal becomes occluded in the shrunken support medium with a loss of exposed catalyst surface area and a corresponding decrease in catalytic activity. It is a known expedient in the art to stabilize alumina supports against such thermal degradation by the use of materials such as zirconia, titania, alkaline earth metal oxides such as baria, calcia or strontia or, most usually, rare earth metal oxides, for example, ceria, lanthana and mixtures of two or more rare earth metal oxides. For example, see C. D. Keith et al U.S. Pat. No. 4,171,288.
Nonetheless, thermal degradation of the alumina support is a problem which adversely affects the performance and durability of TWC and oxidation catalysts and which is exacerbated by the use of high A/F ratios, often employed in automobile engines, which cause increased oxygen concentrations in the exhaust gases. The use of high A/F ratios improves the fuel economy of automobile engines, but the presence of excess oxygen in the exhaust, referred to in the art as a "lean exhaust", reduces the activity of platinum group metal catalysts, as platinum dispersed on activated alumina support is more readily sintered at elevated temperatures in a lean exhaust atmosphere, thus reducing the available metal surface area of the catalyst.
It is known that bulk cerium oxide (ceria) provides an excellent refractory oxide support for platinum group metals other than rhodium, and enables the attainment of highly dispersed, small crystallites of platinum on the ceria particles, and that the bulk ceria may be stabilized by impregnation with a solution of an aluminum compound, followed by calcination. For example, see U.S. Pat. No. 7,714,694 of C. Z. Wan et al. which discloses aluminum-stabilized bulk ceria, optionally combined with an activated alumina, to serve as a refractory oxide support for platinum group metal components impregnated thereon. The use of bulk ceria as a catalyst support for platinum group metal catalysts other than rhodium, optionally, an aluminum-stabilized bulk ceria as disclosed in aforesaid U.S. Pat. No. 4,714,694, is also disclosed in U.S. Pat. No. 4,727,052 of C. Z. Wan et al. Each of the aforesaid C. D. Keith and C. Z. Wan et al U.S. Patents are also assigned to the assignee of this application.
U.S. Pat. No. 4,708,946 of Ohata et al discloses a three-way conversion catalyst comprising a monolithic honeycomb carrier on which is deposited an alumina-modified cerium oxide material and at least one precious metal selected from the group consisting of platinum, palladium and rhodium, together with activated alumina. The cerium oxide material is obtained by impregnating a water-insoluble cerium compound, such as cerium oxide, cerium hydroxide or cerium carbonate, in finely divided particle form with a water-soluble aluminum compound and/or aluminum hydrates, for example, alumina nitrate (see column g, lines 36-53). The impregnated cerium compound is dried and then calcined in air at a temperature in the range of 300.degree. to 700.degree. C. to provide an alumina-modified cerium oxide. As disclosed at column 3, line 51 through column 4, line 9, the alumina-modified cerium oxide may have dispersed thereon at least one precious metal selected from the group consisting of platinum and palladium and mag contain active alumina on which rhodium may be dispersed.
U.S Pat. No. 4,299,734 of Fugitani et al discloses a three-wag conversion catalyst comprising platinum and palladium or mixtures thereof supported on a porous zirconia carrier which contains, per liter of carrier, from about 1 to 80 grams of at least one of cerium oxide, manganese oxide and iron oxide (column 1, lines 49-57 and column 2, lines 17-24). At column 2, lines 39-64, impregnation of the zirconia carrier with a solution of a salt of cerium, manganese or iron is disclosed as one technique for making the catalyst. At lines 57.GAMMA.of column 2, an alternate manufacturing technique is disclosed in which a zirconia powder is mixed with an oxide powder containing at least one of cerium oxide, manganese oxide or iron oxide powder, and the mixed powders are sintered. When sintering the mixed zirconia and oxide powders (e.g., cerium oxide powder), about 1 to 5 percent by weight of alumina is added as a binder (column 2, line 65 to column 3, line 2).
Both platinum and palladium catalytic components are included on a ceria-impregnated zirconia carrier in the catalyst denominated A7 in Table 2 of the Fujitani Patent.